Effects of intracolonic opioid receptor agonists on polymodal pelvic nerve afferent fibers in the rat

Recommendations for evaluation of bladder and bowel function in pre-clinical spinal cord injury research

Objective: In order to encourage the inclusion of bladder and bowel outcome measures in preclinical spinal cord injury (SCI) research, this paper identifies and categorizes 1) fundamental, 2) recommended, 3) supplemental and 4) exploratory sets of outcome measures for pre-clinical assessment of bladder and bowel function with broad applicability to animal models of SCI.Methods: Drawing upon the collective research experience of autonomic physiologists and informed in consultation with clinical experts, a critical assessment of currently available bladder and bowel outcome measures (histological, biochemical, in vivo functional, ex vivo physiological and electrophysiological tests) was made to identify the strengths, deficiencies and ease of inclusion for future studies of experimental SCI.Results: Based upon pre-established criteria generated by the Neurogenic Bladder and Bowel Working Group that included history of use in experimental settings, citations in the literature by multiple independent groups, ease of general use, reproducibility and sensitivity to change, three fundamental measures each for bladder and bowel assessments were identified. Briefly defined, these assessments centered upon tissue morphology, voiding efficiency/volume and smooth muscle-mediated pressure studies. Additional assessment measures were categorized as recommended, supplemental or exploratory based upon the balance between technical requirements and potential mechanistic insights to be gained by the study.Conclusion: Several fundamental assessments share reasonable levels of technical and material investment, including some that could assess bladder and bowel function non-invasively and simultaneously. Such measures used more inclusively across SCI studies would advance progress in this high priority area. When complemented with a few additional investigator-selected study-relevant supplemental measures, they are highly recommended for research programs investigating the efficacy of therapeutic interventions in preclinical animal models of SCI that have a bladder and/or bowel focus.

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.

Pharmacological, Pharmacokinetic, and Pharmacogenomic Aspects of Functional Gastrointestinal Disorders

This article reviews medications commonly used for the treatment of patients with functional gastrointestinal disorders. Specifically, we review the animal models that have been validated for the study of drug effects on sensation and motility; the preclinical pharmacology, pharmacokinetics, and toxicology usually required for introduction of new drugs; the biomarkers that are validated for studies of sensation and motility endpoints with experimental medications in humans; the pharmacogenomics applied to these medications and their relevance to the FGIDs; and the pharmacology of agents that are applied or have potential for the treatment of FGIDs, including psychopharmacologic drugs.

Opioid-induced bowel dysfunction

OPINION STATEMENT

Opioid analgesics are commonly prescribed for moderate to severe pain. Opioids exert effects via receptors in the central and enteric nervous systems. Thus, central opioid analgesia can be limited by side effects involving the gastrointestinal tract, particularly by gastrointestinal motility delay. Opioid-induced bowel dysfunction is commonly treated with bulking agents, stimulant laxatives, lubiprostone, and tegaserod (removed from the market in March 2007). However, these treatments' efficacy in opioid bowel dysfunction has not been proven. Recent research has focused on developing peripheral μ opioid antagonists such as methylnatrexone and alvimopan. These drugs selectively block μ opioid receptors in the enteric nervous system without penetrating the blood-brain barrier and can avert adverse gastrointestinal symptoms of opioids without reducing central analgesia. Methylnaltrexone and alvimopan also reduce hospitalization duration in surgical patients with postoperative ileus. A second line of research has focused on peripheral κ opioid agonists that modulate nociception in the enteric nervous system without producing central nervous system side effects. Asimadoline and fedotozine reduce nociceptive reflexes caused by gut distention and improve pain symptoms in patients with irritable bowel syndrome. ADL 10-0101 (Adolor Corp., Exton, PA) is another peripheral κ opioid agonist that lowers pain scores in patients with chronic pancreatitis. Although peripheral κ opioid agonists are promising, clinical studies are needed to assess their efficacy in treating opioid-induced bowel dysfunction.

Kappa opioids and the modulation of pain

BACKGROUND AND RATIONALE

Pain is a complex sensory experience, involving cognitive factors, environment (setting, society, and culture), experience, and gender and is modulated significantly by the central nervous system (CNS). The mechanisms by which opioid analgesics work are understood, but this class of drugs is not ideal as either an analgesic or anti-hyperalgesic. Accordingly, considerable effort continues to be directed at improved understanding of nociceptor function and development of selective analgesics that do not have the unwanted effects associated with opioid analgesics.

OBJECTIVE

The purpose of this paper is to provide a review of the role of KOP receptors in the modulation of pain and highlight several chemotypes currently being explored as peripherally restricted KOP ligands.

RESULTS

A growing body of literature has shown that KOP receptors are implicated in a variety of behavioral pain models. Several different classes of peripherally restricted peptidic and nonpeptidic KOP agonists have been identified and show utility in treating painful conditions.

CONCLUSION

The pharmacological profile of KOP agonists in visceral pain models suggest that peripherally restricted KOP agonists are potentially useful for a variety of peripheral pain states. Further, clinical investigation of peripherally restricted KOP agonists will help to clarify the painful conditions where KOP agonists will be most effective.

Involvement of kappa-opioid receptors in visceral nociception in mice

It has been shown that the behavioural responses to chemically evoked visceral nociception are increased in transgenic mice lacking the kappa-opioid receptor (KOR). The aim of the present study was to evaluate the contribution of KOR in mechanically evoked visceral pain by performing colorectal distension (CRD) and monitoring the subsequent visceromotor response (VMR) in control mice (KOR(+/+)) and in mice lacking KOR (KOR(-/-)). Pseudo-affective visceral pain responses were evoked in conscious mice using increasing (10-80 mmHg) and repeated (12 x 55 mmHg) phasic CRD paradigms. The resulting VMR was determined by monitoring the electromyographic activity of the abdominal muscle. The increasing and repeated CRD paradigms, respectively, evoked similar responses in both KOR(+/+) and KOR(-/-) mice. The selective KOR-agonists U-69593 (5 and 25 mg kg(-1), s.c.) and asimadoline (25 mg kg(-1), s.c.) significantly decreased the VMR in KOR(+/+) mice, while having no effect in KOR(-/-) mice. In contrast, the selective mu-opioid receptor agonist fentanyl significantly reduced the VMR in both types of mice and appeared more efficacious in KOR(-/-) mice. The opioid receptor antagonist naloxone (0.3-30 mg kg(-1) s.c.) did not affect the response to CRD in C57BL/6 mice at any dose tested. In conclusion, the data confirm that the KOR agonists used in this study inhibit the VMR to CRD in mice by acting via KOR receptors. In addition, the data suggest that the endogenous opioid system is not likely to modulate the VMR to mechanically evoked visceral pain in mice.

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.

Opioid modulation of ferret vagal afferent mechanosensitivity

Despite universal use of opioids in the clinic to inhibit pain, there is relatively little known of their peripheral actions on sensory nerve endings, where in fact they may be better targeted with more widespread applications. Here we show differential effects of mu-, kappa-, and delta-opioids on mechanosensitive ferret esophageal vagal afferent endings investigated in vitro. The effects of selective agonists [d-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO), 2-(3, 4-dichlorophenyl)-N-methyl-N-[(1S)-1phenyl-2-(1-pyrrolidinyl) ethyl] acetamide hydrochlorine (ICI 199441), and (+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80), respectively, on mechanosensory stimulus-response functions were quantified. DAMGO (10(-7) to 10(-5) M) reduced the responses of tension receptors to circumferential tension (1-5 g) by up to 50%, and the responses of mucosal receptors to mucosal stroking (10-1,000 mg von Frey hair) by >50%. DAMGO effects were reversed by naloxone (10(-5) M). Tension/mucosal (TM) receptor responses to tension and stroking were unaffected by DAMGO. ICI 199441 (10(-6) to 10(-5) M) potently inhibited all responses except TM receptor responses to tension, and SNC-80 (10(-5) to 10(-3) M) had no effect other than a minor inhibition of mucosal receptor responses to intense stimuli at 10(-3) M. We conclude that mu- and kappa-opioids have potent and selective peripheral effects on esophageal vagal afferents that may have applications in treatment of disorders of visceral sensation.

Pharmacological and pharmacokinetic aspects of functional gastrointestinal disorders

Medications are commonly used for the treatment of patients with functional gastrointestinal disorders. The general goal of this report is to review the pharmacokinetics and pharmacology of medications used in functional gastrointestinal disorders. Methods included literature review, consensus evaluation of the evidence for each topic assigned originally to 1 or 2 authors, and broader review at a harmonization session as part of the Rome III process. This report reviews the animal models that have been validated for the study of effects of pharmacologic agents on sensation and motility; the preclinical pharmacology, pharmacokinetics, and toxicology usually required for introduction of novel therapeutic agents; the biomarkers validated for studies of sensation and motility end points with experimental medications in humans; the pharmacogenomics applied to these medications and disorders; and the pharmacology of agents that are applied or have potential for treatment of functional gastrointestinal disorders, including psychopharmacologic agents. Clinician and basic investigators involved in the treatment or investigation of functional gastrointestinal disorders or disease models need to have a comprehensive understanding of a vast range of medications. It is anticipated that the interaction between investigators of basic science, basic and applied pharmacology, and clinical trials will lead to better treatment of these disorders.

Irritable bowel syndrome: new and emerging therapies

PURPOSE OF REVIEW

Irritable bowel syndrome refers to abdominal discomfort associated with altered bowel habits. Recent evidence suggests that the primary pathophysiologic mechanism is brain-gut dysregulation. Many central and peripheral factors are involved. This article will review important pathophysiologic mechanisms with a focus on new and emerging therapies.

RECENT FINDINGS

Prior gastroenteritis and small intestinal bacterial overgrowth may be important for treatment of irritable bowel syndrome. Understanding of serotonergic receptors in gastrointestinal function has led to the development of serotonergic agents such as alosetron and tegaserod. Novel agents targeting other receptor sites include neurokinin and neurohormonal modulators, chloride channels and opioid receptors. Other therapeutic approaches - behavioral treatments, probiotics, antibiotics and alternative therapies - have developing roles in the treatment of irritable bowel syndrome.

SUMMARY

A better understanding of pathophysiologic mechanisms has resulted in therapeutic advances. Prokinetic therapies may have a role in nondiarrhea predominant irritable bowel syndrome. Antidepressants are used to modulate pain and treat comorbid psychological distress. Newer agents target various receptor sites. Advances in psychological/behavioral treatments and alternative modalities hold promise for the future.

Morphine directly inhibits nociceptors in inflamed skin

Peripherally delivered opiates attenuate mechanical and thermal hyperalgesia in experimental models of inflammation, suggesting that activation of peripheral opioid receptors decreases the excitability of nociceptors in inflamed tissues. The current study examines the effects of peripheral morphine sulfate on response properties of sensory neurons in healthy and inflamed skin. Afferent units (185) were isolated from tibial nerve of rats using an in vitro glabrous skin-nerve teased-fiber preparation. Of these, 107 units were from normal healthy skin, and 78 were from inflamed skin 18 h after intraplantar injection of complete Freund's adjuvant. As a population, C-fiber units innervating inflamed skin exhibited properties characteristic of sensitization when compared with units innervating healthy control skin. Mechanical thresholds were lowered, responses to noxious mechanical and thermal stimuli were elevated, a greater proportion of units was spontaneously active, and the average rate of spontaneous discharge was higher. Response properties in other conduction velocity groups remained unchanged. Fifty-eight percent of C and C/Adelta nociceptors innervating inflamed skin were opiate-sensitive, and their excitability was attenuated by direct application of morphine to their receptive fields. All morphine-sensitive units were nociceptors from inflamed skin with conduction velocities <1.3 m/s. Morphine effects were concentration-dependent and naloxone-sensitive, indicating that the effects were receptor-mediated. These findings provide direct evidence that morphine acts through peripheral opioid receptors to inhibit the activity of cutaneous nociceptors under conditions of inflammation.

Evaluation of pseudo-affective responses to noxious colorectal distension in rats by manometric recordings

Recordings of electromyographic (EMG) activity in the abdominal musculature are generally used to quantify the pseudo-affective visceromotor response induced by colorectal distension (CRD) in rodents. The present study describes a non-invasive, manometric method to quantify the magnitude of the abdominal contractions evoked by CRD. CRD-induced increases in EMG activity in female rats (electrical response) were compared to phasic changes in balloon pressure (mechanical response). A phasic increasing CRD paradigm from 10 to 80mmHg with 10mmHg intervals induced a clear stimulus-response relationship with a strong correlation (r(2)=0.93) between the electrical and mechanical responses. Twelve repeated phasic distensions at 80mmHg increased the mechanical response by 133+/-53% (P<0.01), while the electrical response only increased by 20+/-19% (P>0.05), when comparing the last distension to the first. Atropine methyl bromide (1mg/kg, i.v.) did not affect the mechanical response to distension at 80mmHg, suggesting that colonic activity per se, does not contribute to the balloon pressure variations during CRD in the current experimental set-up. The mu-opioid receptor agonist fentanyl at a dose of 1.5microg/kg (i.v.) significantly reduced the mechanical response to CRD (P<0.01) while the electrical response was not affected. The present study shows that phasic bursts in EMG activity from the abdominal musculature occur simultaneously with balloon pressure variations, which may represent a non-invasive alternative to EMG recordings. Furthermore, the mechanical response is a more sensitive parameter for detecting both hyperalgesic and analgesic responses.

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.

Chronic Estrogen Sensitizes a Subset of Mechanosensitive Afferents Innervating the Uterine Cervix

Estrogen increases reflex nocifensive responses to distension of the uterus and the urinary bladder, but estrogen's effects on afferent response to distension of the uterine cervix, the site of obstetric and some gynecologic pain, has not been studied. Here, single fiber recording of hypogastric nerve responses to uterine cervical distension were obtained from ovariectomized (OVX) rats and OVX rats treated with estrogen (ES). Spontaneous activity was greater in the ES group (13 of 24 units; 54%) than in the OVX group (6 of 27 units; 22%). ES differentially altered the response of low- and high-threshold units to distension. For high-threshold units, firing frequency was increased two- to fourfold with 60–100 gm distension in ES compared with OVX groups ( P < 0.05). In contrast, the response of low-threshold units to distension was not altered by ES. About one-half of units tested in each group responded to a temperature increase from 35 to 49°C. A greater proportion of thermosensitive units were also mechanosensitive in the ES group (7 of 8 afferents, 88%) than in the OVX group (5 of 11 afferents, 45%). Acute application of ES in OVX rats failed to evoke or increase distension-induced responses. These data show the polymodal nature of afferent fibers innervating the uterine cervix. Increased spontaneous activity with ES may play a part in remodeling of the cervical tissue, whereas selective sensitization of high-threshold units by ES might underlie increased pain responses to cervical distension. Failure of acute ES treatment to mimic this suggests a genomic effect.

Peripheral opiate action on afferent fibres supplying the rat intestine

The aim of the present study was to examine the sensitivity of mesenteric afferents supplying the rat small intestine to mu-opioid receptor ligands. Mesenteric afferent discharge was recorded electrophysiologically in response to [D-ALA2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO; 100 mug kg(-1) i.v.), before and after treatment with the mu-receptor antagonist alvimopan (1 mg kg(-1) i.v.). DAMGO markedly stimulated whole nerve mesenteric afferent discharge (P < 0.05), an effect completely blocked by alvimopan. The response of mesenteric afferents to 2-methyl-5-hydroxytryptamine (30 microg kg(-1) i.v.), bradykinin (0.1-1 microg kg(-1) i.a.) and both low- and high-threshold distension (0-60 mmHg) was unaffected by alvimopan. In chronically vagotomized animals, the low-threshold response to distension was attenuated while the remaining high-threshold response was unaffected by alvimopan. In conclusion, mesenteric afferent fibres are markedly stimulated by mu-opioid receptor agonists, an effect blocked by alvimopan, which may contribute to the gastrointestinal reflex and behavioural responses to opiate treatment or abuse. However, alvimopan did not influence the normal sensitivity of intestinal afferents to chemical and mechanical stimuli that activate different subpopulations of vagal and spinal afferents. Thus, alvimopan may be useful for the treatment of gastrointestinal sequelae following opiate treatment for postoperative or chronic pain.

Treating irritable bowel syndrome: overview, perspective and future therapies

This article summarizes the ongoing challenges in irritable bowel syndrome and the exciting opportunities for development of novel therapies for this common, enigmatic condition. The challenges include insufficient understanding of mechanisms, lack of specificity of symptoms, differentiation from other conditions, and lack of availability of noninvasive tests to identify dysfunctions. However, significant opportunities are reflected by the advances in clinical trial design and, particularly, clinically relevant end points for such trials, and the increasing understanding of basic neuroenteric science. The latter has delivered two new medications to the practice (alosetron and tegaserod), and other candidate therapies (other serotonergic, tachykininergic, opioid, cannabinoid modulators) are being carefully appraised as potential drugs for the future.

The enteric nervous system III: a target for pharmacological treatment

The past decade has seen major advances in the pharmacological understanding of the nervous system of the gastrointestinal tract, the enteric nervous system, and its importance for gut functions in several states of disease. Indeed, the enteric nervous system has become a promising target in the treatment of many gastrointestinal symptoms and disorders. Some of these new therapeutic concepts, such as botulinum toxin for achalasia and serotonergic drugs for functional bowel diseases, are already in clinical use. This paper is part 3 of three Minireviews in Pharmacology & Toxicology, and presents the neurogastrointestinal pharmacological therapeutic options in gastrointestinal pain, functional gastrointestinal disorders, inflammatory bowel diseases, cancer and related conditions with focus on future drug targets. The diagnosis of gastrointestinal neuropathy, the role of serotonin and related neuroendocrine transmitters, serotonergic drugs, and neurotrophic factors in neurogastrointestinal pharmacology will be addressed in this context.

Nonopioid actions of U50,488 enantiomers contribute to their peripheral cutaneous antinociceptive effects

The ability of arylacetamide kappa-opioid receptor agonists (kappa-ORAs) to block sodium channels by a nonopioid mechanism has been previously documented. The present experiments were undertaken to test whether two enantiomers of the arylacetamide kappa-ORA (trans)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]benzeneacetamide (U50,488), (+)-(1R,2R)-U50,488 and (-)-(1S,2S)-U50,488, are antinociceptive in the formalin model by a peripheral, nonopioid receptor-mediated mechanism. Although both enantiomers have been previously shown to block sodium channels with comparable potencies, only (-)-(1S,2S)-U50,488 has activity at the kappa-opioid receptor (KOR). In the formalin test, intrapaw administration of U50,488 enantiomers as well as lidocaine exhibited significant dose-related attenuation of formalin-induced flinching behavior. The rank order of potency of the drugs tested was (-)-(1S,2S)-U50,488 > (+)-(1R,2R)-U50,488 > lidocaine. The antinociception produced by lower doses of (-)-(1S,2S)-U50,488 was blocked by intrapaw nor-binaltorphimine as well as by antisense knockdown of the KOR. Such pretreatments, however, did not block the antinociception produced by (+)-(1R,2R) U50,488, lidocaine, or higher doses of (-)-(1S,2S)-U50,488. These data suggest that the sodium channel blocking effects of U50,488 and similar kappa-ORAs can contribute to their peripheral antinociceptive actions.

Changes of mu and kappa opioid receptors in cathartic colon of rats

AIM

To study the changes of mu and kappa opioid receptors in cathartic colon of rats.

METHODS

An experimental rat model of cathartic colon was reconstructed by contact laxatives mixed with the feed. The activity of mu, kappa opioid receptors in cathartic colon of rats was determined.

RESULTS

The maximal binding capacity (Bmax) and affinity (Kd) of mu opioid receptor in cathartic colon of rats were significantly increased as compared with the control group(207.00±22.9 vs 82.00±14.23 fmol/mg.P, P<0.01; 3.300.45 vs 2.40±0.57 mmol/L, P<0.05). As compared with the control group, the maximal binding capacity of kappa opioid receptor was significantly increased (957.00±102.41 vs 459.00±52.41 fmol/mg.P, P<0.01), and no remarkable change of affinity was found in cathartic colon of rats.

CONCLUSION

The data indicates that mu and kappa opioid receptors may be involved in the disorder of cathartic colon, but the mu opioid receptor may be more important.

Effects of a kappa-opioid agonist, asimadoline, on satiation and GI motor and sensory functions in humans

To compare the effects of the kappa-opioid agonist asimadoline and placebo on visceral sensation and gastrointestinal (GI) motor functions in humans, 91 healthy participants were randomized in a double-blind fashion to 0.15, 0.5, or 1.5 mg of asimadoline or placebo orally twice a day for 9 days. We assessed satiation (nutrient drink test), colonic compliance, tone, perception of colonic distension (barostat), and whole gut transit (scintigraphy). Treatment effect was assessed by analysis of covariance. Asimadoline increased nutrient drink intake (P = 0.03). Asimadoline decreased colonic tone during fasting (P = 0.03) without affecting postprandial colonic contraction, compliance, or transit. Gas scores in response to colonic distension were decreased with 0.5 mg of asimadoline at low levels (8 mmHg above operating pressure) of distension (P = 0.04) but not at higher levels of distension. Asimadoline at 1.5 mg increased gas scores at 16 mmHg of distension (P = 0.03) and pain scores at distensions of 8 and 16 mmHg (P = 0.003 and 0.03, respectively) but not at higher levels of distension. Further studies of this compound in diseases with altered satiation or visceral sensation are warranted.

Effect of GABA(B) receptor agonist on distension-sensitive pelvic nerve afferent fibers innervating rat colon

Spinal afferents innervating the gastrointestinal tract are the major pathways for visceral nociception. Many centrally acting analgesic drugs attenuate responses of visceral primary afferent fibers by acting at the peripheral site. Gamma-amino butyric acid (GABA), a major inhibitory neurotransmitter, acts via metobotropic GABA(B) and ionotropic GABA(A)/GABA(C) receptors. The aim of this study was to test the peripheral effect of selective GABA(B) receptor agonist baclofen on responses of the pelvic nerve afferent fibers innervating the colon of the rat. Distension-sensitive pelvic nerve afferent fibers were recorded from the S(1) sacral dorsal root in anesthetized rats. The effect of baclofen (1-300 micromol/kg) was tested on responses of these fibers to colorectal distension (CRD; 60 mmHg, 30 s). A total of 21 pelvic nerve afferent fibers was recorded. Mechanosensitive properties of four fibers were also recorded before and after bilateral transections of T(12)-S(3) ventral roots (VR). Effect of baclofen was tested on 15 fibers (7 in intact rats, 4 in rats with transected VR, and 4 in rats pretreated with CGP 54626). In nine fibers (5/7 in intact and 4/4 in VR transected rats), baclofen produced dose-dependent inhibition of response to CRD. Pretreatment with selective GABA(B) receptor antagonist CGP 54626 (1 micromol/kg) reversed the inhibitory effect of baclofen. Results suggest a peripheral role of GABA(B) receptors in the inhibition of mechanotransduction property of distension-sensitive pelvic nerve afferent fibers.

Sodium channel blocking actions of the kappa-opioid receptor agonist U50,488 contribute to its visceral antinociceptive effects

The goal of the present study was to determine whether the kappa-opioid receptor agonist (ORA) U50,488 attenuates behavioral and primary afferent nerve responses to noxious colorectal distension (CRD) by sodium channel blockade. We tested the analgesic kappa-ORA (+/-)-trans U50,488, its enantiomers (-)-trans (1S,2S)-U50,488 and non kappa-ORA (+)-trans (1R,2R)-U50,488, and/or its diastereomer (-)-cis (1S,2R)-U50,488 for their ability to attenuate visceromotor and pelvic nerve afferent fiber responses to noxious CRD in vivo and voltage-activated sodium current in colon sensory neurons in vitro. In unanesthetized rats, subcutaneous administration of U50,488, (1S,2S)-U50,488, and (1R,2R)-U50,488 attenuated the behavioral visceromotor response to noxious CRD; the rank order of potency was: (1S,2S)-U50,488 > U50,488 angle quotation mark, right (1R,2R)-U50,488. U50,488 and its stereoisomers also inhibited responses of decentralized pelvic nerve afferent fibers to noxious CRD in a dose-dependent manner. Cumulative doses of 16 mg/kg of (1S,2S)-U50,488, (1S,2R)-U50,488, and (1R,2R)-U50,488 reduced responses to a mean 29, 30, and 47% of control, respectively. The mean inhibitory doses of these drugs were not different (range: 6.6-10.8 mg/kg). Sodium channel blockers mexiletine and carbamazepine mimicked the effect of U50,488. In contrast, the kappa-ORAs dynorphin (1-13) and ICI 204,488 were ineffective in attenuating pelvic nerve activity. Perfusion of (1S,2S)-U50,488, (1S,2R)-U50,488, or (1R,2R)-U50,488 on colon sensory neurons in vitro decreased voltage-activated sodium currents. This inhibition by U50,488 and its stereoisomers was not opioid receptor-mediated because it could not be reversed by the opioid receptor antagonist naloxone and was also not a G protein-mediated effect. The results reported here suggest that the visceral antinociceptive effects of U50,488 and its stereoisomers are contributed to by their peripheral sodium channel blocking actions.

Effect of kappa opioid agonists on visceral nociception induced by uterine cervical distension in rats

Although uterine distension in rats results in an escape reflex, there exists no model of uterine cervical distension (UCD), the pain stimulus during the first stage of labor. The aims of this study were to develop such a model in virgin rats and to test whether peripherally restricted kappa opioid receptor (KOR) agonists (ADL 10-0101, ADL 10-0102, ADL 10-0116) inhibit responses to UCD. Under intravenous (i.v.) pentobarbital and alpha-chloralose anesthesia, fine metal rods were inserted in both uterine cervical osses through a small midline laparotomy. UCD was performed by manual separation of the rods (25-100 g). Single-unit afferent responses in hypogastric nerve or reflex rectus abdominis electromyographic (EMG) activity were determined before and after i.v. KOR agonists. UCD resulted in a stimulus-dependent increase in single-unit afferent activity. Units could be characterized as low threshold (mean threshold 6.6+/-2.7 g), or high threshold (mean threshold 55+/-8.8 g); all were C fibers, all responded to topical bradykinin. ADL 10-0116 (10 mg/kg) reduced the afferent response to UCD. Reflex EMG response occurred over a distension force range of 25-100 g, unaffected by i.v. saline. All three KOR agonists produced a dose-dependent, naloxone-reversible inhibition of the EMG response with a potency relationship of ADL 10-0102 (ED50 0.04 mg/kg)>ADL 10-0101 (ED50 0.65 mg/kg)=ADL 10-0116 (ED50 0.60 mg/kg). These data support the use of acute UCD as a noxious stimulus, inducing afferent and reflex activity. Like other visceral stimuli, UCD is sensitive to inhibition by KOR agonists.

Endogenous opiates: 2000

This paper is the twenty-third installment of the annual review of research concerning the opiate system. It summarizes papers published during 2000 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Modulation of CGRP and PGE2 release from isolated rat skin by alpha-adrenoceptors and kappa-opioid-receptors

Norepinephrine (NE) reduces the release of neuropeptides from central terminals of primary afferent neurones by presynaptic inhibition. We investigated whether NE also affects stimulus-induced intracutaneous calcitonin gene-related peptide (CGRP) and secondary prostaglandin E2 (PGE2) release. For comparison, kappa-opioid effects were examined. Antidromic electrical nerve stimulation resulted in significant increases in the release of CGRP and PGE2. The PGE2 release was prevented by selective activation of alpha2-adrenoceptors whereas the CGRP release was not changed. In contrast, selective kappa-opioid receptor activation diminished electrically evoked release of both CGRP and PGE2. We conclude that NE affected stimulated PGE2 release via alpha2-adrenoceptors on cells other than cutaneous afferents while kappa-opioid receptors are expressed in peripheral terminals of cutaneous afferents and their activation reduced CGRP release and secondary PGE2 formation.

Visceral pain

Visceral pain, although different from somatic pain in several important features, is not as widely researched and consequently our knowledge of neurophysiologic mechanisms as well as clinical management of visceral pain states remains unsatisfactory. Several recent studies have employed different visceral pain animal models to provide insight into the peripheral and central nervous system mechanisms underlying pain originating from the urinary bladder, ureter, and gastrointestinal tract. The effects of opioid and nonopioid drugs in these models have also been evaluated and are reviewed in this article. The importance of anatomic pathways relaying pain sensation in the central nervous system, particularly the newly described dorsal column pathway, is also discussed. In human subjects, new techniques like positron emission tomography are now being used to better understand visceral pain perception. Such findings deriving from basic animal research and human studies summarized in the present overview lead to a better understanding of visceral pain states and may be helpful in developing better treatment strategies to combat visceral pain states in the clinical setting.

Peripheral opioid modulation of visceral pain

Opioids are widely and successfully used for control of pain, including pain arising from the viscera. Constipating, sedating, respiratory depressant and other effects of opioids, however, often limit their long-term use in the treatment of a variety of visceral pain states. Accordingly, understanding visceral pain mechanisms and its modulation is important to developing improved strategies for pain control.

kappa -opioid receptor agonists modulate visceral nociception at a novel, peripheral site of action

kappa-opioid receptor agonists (kappa-ORAs) have been shown to modulate visceral nociception through an interaction with a peripheral, possibly novel, kappa-opioid-like receptor. We used in the present experiments an antisense strategy to further explore the hypothesis that kappa-ORA effects in the colon are produced at a site different from the cloned kappa-opioid receptor (KOR). An antisense oligodeoxynucleotide (ODN) to the cloned rat KOR was administered intrathecally (12.5 microg, twice daily for 4 d) to specifically knock-down the cloned KOR. Efficacy of the KOR antisense ODN treatment was behaviorally evaluated by assessing the antinociceptive effects of peripherally administered kappa- (EMD 61, 753 and U 69,593), mu- (DAMGO) and delta- (deltorphin) ORAs in the formalin test. Intrathecal antisense, but not mismatch ODN blocked the actions of EMD 61,753 and U 69,593 without affecting the actions of DAMGO or deltorphin; a complete recovery of antinociceptive actions of the kappa-ORA EMD 61,753 was observed 10 d after the termination of antisense ODN treatment. In contrast, the ability of EMD 61,753 to dose-dependently attenuate responses of pelvic nerve afferent fibers to noxious colonic distension was unaffected in the same rats in which the antisense ODN effectively knocked-down the KOR as assessed in the formalin test. Additionally, Western blot analysis demonstrated a significant downregulation of KOR protein in the L4-S1 dorsal root ganglia of antisense, but not mismatch ODN-treated rats. The present results support the existence of a non-kappa-opioid receptor site of action localized in the colon.