Role of vagal afferents in the antinociception produced by morphine and U-50,488H in the colonic pain reflex in rats

Inhibition of Canonical Transient Receptor Potential Channels 4/5 with Highly Selective and Potent Small-Molecule HC-070 Alleviates Mechanical Hypersensitivity in Rat Models of Visceral and Neuropathic Pain

Transient receptor potential channels C4/C5 are widely expressed in the pain pathway. Here, we studied the putative analgesic efficacy of the highly selective and potent TRPC4/C5 antagonist HC-070 in rats. Inhibitory potency on human TRPC4 was assessed by using the whole-cell manual patch-clamp technique. Visceral pain sensitivity was assessed by the colonic distension test after intra-colonic trinitrobenzene sulfonic acid injection and partial restraint stress. Mechanical pain sensitivity was assessed by the paw pressure test in the chronic constriction injury (CCI) neuropathic pain model. We confirm that HC-070 is a low nanomolar antagonist. Following single oral doses (3-30 mg/kg in male or female rats), colonic hypersensitivity was significantly and dose-dependently attenuated, even fully reversed to baseline. HC-070 also had a significant anti-hypersensitivity effect in the established phase of the CCI model. HC-070 did not have an effect on the mechanical withdrawal threshold of the non-injured paw, whereas the reference compound morphine significantly increased it. Analgesic effects are observed at unbound brain concentrations near the 50% inhibitory concentration (IC₅₀) recorded in vitro. This suggests that analgesic effects reported here are brought about by TRPC4/C5 blocking in vivo. The results strengthen the idea that TRPC4/C5 antagonism is a novel, safe non-opioid treatment for chronic pain.

Vagal Interoceptive Modulation of Motivated Behavior

In addition to regulating the ingestion and digestion of food, sensory feedback from gut to brain modifies emotional state and motivated behavior by subconsciously shaping cognitive and affective responses to events that bias behavioral choice. This focused review highlights evidence that gut-derived signals impact motivated behavior by engaging vagal afferents and central neural circuits that generally serve to limit or terminate goal-directed approach behaviors, and to initiate or maintain behavioral avoidance.

Effect of estrogen on vagal afferent projections to the brainstem in the female

The effects of 17β-estradiol (E) on the distribution and density of brainstem projections of small or large diameter primary vagal afferents were investigated in Wistar rats using transganglionic transport of wheat germ agglutinin- (WGA; preferentially transported by non-myelinated afferent C-fibers; 2%), or cholera toxin B-subunit- (CTB, 5%; preferentially transported by large myelinated afferent A-fibers) conjugated horseradish peroxidase (HRP) in combination with the tetramethylbenzidine method in age matched ovariectomized (OVX) only or OVX and treated with E (OVX+E; 30 pg/ml plasma) females for 12 weeks. Additionally, these projections were compared to aged matched males. Unilateral microinjection of WGA-HRP into the nodose ganglion resulted in dense anterograde labeling bilaterally, with an ipsilateral predominance in several subnuclei of the nucleus of the solitary tract (NTS) and in area postrema that was greatest in OVX+E animals compared to OVX only and males. Moderately dense anterograde labeling was also observed in paratrigeminal nucleus (PAT) of the OVX+E animals. CTB-HRP produced less dense anterograde labeling in the NTS complex, but had a wider distribution within the brainstem including the area postrema, dorsal motor nucleus of the vagus, PAT, the nucleus ambiguus complex and ventrolateral medulla in all groups. The distribution of CTB-HRP anterograde labeling was densest in OVX+E, less dense in OVX only females and least dense in male rats. Little, if any, labeling was found within PAT in males using either WGA-or CTB-HRP. Taken together, these data suggest that small, non-myelinated (WGA-labeled) and large myelinated (CTB-labeled) diameter vagal afferents projecting to brainstem autonomic areas are differentially affected by circulating levels of estrogen. These effects of estrogen on connectivity may contribute to the sex differences observed in central autonomic mechanisms between gender, and in females with and without estrogen.

Effects of Bifidobacterium infantis 35624 on post-inflammatory visceral hypersensitivity in the rat

BACKGROUND

Irritable bowel syndrome patients have abnormal visceral perception. Probiotic organisms may produce beneficial effects in these patients by reducing visceral hypersensitivity.

AIM

To investigate the effects of the probiotic organism, Bifidobacterium infantis 35624, on post-inflammatory visceral hypersensitivity in rats.

METHODS

Colitis was induced using intracolonic administration of trinitrobenzenesulfonic acid; control rats received saline (day 0). Myeloperoxidase (MPO) levels and colonic damage scores were determined. From days 15-29, rats (n = 10/group) rats were orally dosed with 2 ml of B. infantis ≥ 10(8) colony-forming units/ml or vehicle (MRS broth). A second series of rats (n = 10/group) was dosed in the same manner from days 15-59. The level of colonic stimulation during colorectal distension (CRD) was determined by recording a visceromotor response (VMR) to CRD at 30 mmHg pre- and post-treatment. Post-treatment samples of colonic tissue were weighed, graded for morphologic damage, and assayed for MPO levels.

RESULTS

All rats were hypersensitive at day 15. On day 30, hypersensitivity to colorectal distension remained in the vehicle group, but was significantly reduced in the B. infantis group (mean VMR/10 min: vehicle = 15.4 ± 1.0 vs. B. infantis = 7.6 ± 1.0, p < 0.001). A similar, significant effect was observed at day 60. On both day 30 and day 60, tissue weight, colonic damage scores, and MPO levels resembled those of control animals.

CONCLUSIONS

Oral administration of Bifidobacterium infantis 35624 normalized sensitivity to colorectal distension in a rat model of post-inflammatory colonic hypersensitivity.

Anxiety increases acid-induced esophageal hyperalgesia

OBJECTIVE

Anxiety at the time of gastrointestinal injury or inflammation increases the risk of developing visceral hyperalgesia. Distal esophageal acidification induces hyperalgesia in the non-acid exposed proximal esophagus, due to the sensitization of spinal dorsal horn neurones. The objective was to determine whether anxiety influences acid-induced hyperalgesia.

METHODS

A total of 19 healthy volunteers (n = 12 females; age, 22-57 years; mean, 35.7 years) completed a 10-minute mood induction paradigm (anxiety or neutral) with autonomic monitoring (visit 1). On visits 2 and 3, pain thresholds to electrical stimulation, in milliamperes (mA), were determined in the proximal esophagus and foot (control) before and after a 30-minute infusion of 0.15 M of hydrochloric acid. During esophageal acid infusion, the subjects randomly received anxiety or neutral mood induction with autonomic monitoring, in a crossover design. Anxiety and pain ratings were recorded pre and post infusion.

RESULTS

Visit 1: Anxiety induction increased anxiety scores (p < .001), mean arterial pressure (p < .001), and cardiac sympathetic index (p = .007), and reduced parasympathetic measures (cardiac vagal tone [p = .05] and cardiac sensitivity to baroreflex [p = .006)]). Visit 2: Anxiety induction conferred greater acid-induced hyperalgesia compared with neutral (-4.9 mA versus 2.7 mA, p = .009, analysis of covariance). No differences in autonomic measures were found during acid infusion with anxiety or neutral mood induction.

CONCLUSIONS

Anxiety induction increases acid-induced esophageal hyperalgesia; anxiety, thus, facilitates central sensitization in the esophagus. Our studies provide a new model for studying the effects of anxiety on esophageal hyperalgesia and may allow testing of therapeutic strategies to reduce this effect.

Morphine modulation of temporomandibular joint-responsive units in superficial laminae at the spinomedullary junction in female rats depends on estrogen status

The influence of analgesic agents on neurons activated by stimulation of the temporomandibular joint (TMJ) region is not well defined. The spinomedullary junction [trigeminal subnucleus caudalis (Vc)/C(1-2)] is a major site of termination for TMJ sensory afferents. To determine whether estrogen status influences opioid-induced modulation of TMJ units, the classical opioid analgesic, morphine, was given to ovariectomized (OvX) rats and OvX rats treated for 2 days with low-dose (LE2) or high-dose (HE2) 17beta-estradiol-3-benzoate. Under thiopental anesthesia, TMJ units in superficial and deep laminae at the Vc/C(1-2) junction were activated by injection of ATP (1 mm) directly into the joint space. In superficial laminae, morphine inhibited evoked activity in units from OvX and LE2 rats in a dose-related and naloxone-reversible manner, whereas units from HE2 rats were not inhibited. By contrast, in deep laminae, morphine reduced TMJ-evoked unit activity similarly in all groups. Morphine reduced the background activity of units in superficial and deep laminae and resting arterial pressure similarly in all groups. Morphine applied to the dorsal surface of the Vc/C(1-2) junction inhibited all units independently of E2 treatment. Quantitative polymerase chain reaction and immunoblots revealed a similar level of expression for mu-opioid receptors at the Vc/C(1-2) junction in LE2 and HE2 rats. These results indicated that estrogen status differentially affected morphine modulation of TMJ unit activity in superficial, but not deep, laminae at the Vc/C(1-2) junction in female rats. The site(s) for estrogen influence on morphine-induced modulation of TMJ unit activity was probably outside the medullary dorsal horn.

Basic and clinical aspects of gastrointestinal pain

The gastrointestinal (GI) tract is a system of organs within multicellular animals which facilitates the ingestion, digestion, and absorption of food with subsequent defecation of waste. A complex arrangement of nerves and ancillary cells contributes to the sensorimotor apparatus required to subserve such essential functions that are with the exception of the extreme upper and lower ends of the GI tract normally subconscious. However, it also has the potential to provide conscious awareness of injury. Although this function can be protective, when dysregulated, particularly on a chronic basis, the same system can lead to considerable morbidity. The anatomical and molecular basis of gastrointestinal nociception, conditions associated with chronic unexplained visceral pain, and developments in treatment are presented in this review.

Review article: drugs interfering with visceral sensitivity for the treatment of functional gastrointestinal disorders--the clinical evidence

At present, the concept of visceral hypersensitivity provides the leading hypothesis regarding the generation of symptoms in functional gastrointestinal disorders. This paper discusses the current clinical evidence for drugs that have been proposed to interfere with visceral sensitivity in functional gastrointestinal disorders. Several possible pharmacological targets have been identified to reduce visceral pain and to reverse the processes underlying the persistence of visceral hypersensitivity. However, most of the available evidence comes from experimental animal models and cannot simply be extrapolated to patients with functional gastrointestinal disorders. In this review, we selected five drug classes that have been shown to exhibit visceral analgesic properties in experimental studies, and of which data were available regarding their clinical efficacy. These included opioid substances, serotonergic agents, antidepressants, somatostatin analogues and alpha(2)-adrenergic agonists. Although clinical trials show a limited benefit, in particular for serotonergic agents, the evidence illustrating that these effects result from normalization of visceral sensation is currently lacking. Therefore, we conclude that the concept of targeting visceral hypersensitivity as a treatment for functional gastrointestinal disorders is still controversial. Future evaluations require patient selection based on the presence of visceral hypersensitivity and application of compounds that exhibit 'true' viscerosensory effects.

Novel developments with selective, non-peptidic kappa-opioid receptor agonists

Despite the recent introduction of a number of new compounds, there has of late been a cooling of interest by pharmaceutical companies in the development of centrally-active, selective kappa opioid agonists for therapeutic purposes. This is reflected in the discontinuation of a number of clinical trials, for reasons that are often not completely clear to outside observers. Spiradoline and enadoline have apparently been abandoned as potential analgesics because they induce dose-limiting central side-effects (i.e., dysphoria) in models of post-surgical pain. The development of niravoline as an aquaretic for the treatment of cirrhosis with ascites and other hyponatraemic disorders has also been halted. Enadoline may yet find some application against ischaemic stroke and severe head injury, presumably in comatose patients in whom psychiatric side-effects are taken to be immaterial, while apadoline and TRK 820 remain in Phase II clinical testing against cancer pain. The peripherally-selective kappa agonists, asimadoline, and the atypical compound, fedotozine, are well-tolerated in man. Results of Phase III trials of fedotozine against irritable bowel syndrome and dyspepsia have, however, ultimately been disappointing, whereas asimadoline is currently in Phase II clinical trials against pain of rheumatic and osteoarthritic origin. The results of these trials are eagerly awaited.

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.

Effect of asimadoline, a kappa opioid agonist, on pain induced by colonic distension in patients with irritable bowel syndrome

BACKGROUND

Visceral hypersensitivity plays a major role in irritable bowel syndrome pathophysiology. Opioid kappa receptors on afferent nerves may modulate it and be the target for new irritable bowel syndrome treatments.

AIM

This study evaluated the effect of the kappa opioid agonist asimadoline on perception of colonic distension and colonic compliance in irritable bowel syndrome patients.

METHOD

Twenty irritable bowel syndrome female patients (Rome II criteria; 40 +/- 13 years) and hypersensitivity to colonic distension (Pain threshold < or = 32 mmHg) were included in a randomized double-blind cross-over trial comparing the effect of a single oral dose of asimadoline 0.5 mg or placebo on sensory thresholds (defined as a constant and sustained sensation) elicited by left colon phasic distension (5 mmHg steps, 5 min) up to a sensation of abdominal pain. Colonic compliance was compared by the slope of the pressure-volume curves.

RESULTS

On asimadoline, pain threshold (mean +/- s.d.) (29.8 +/- 7.2 mmHg) was higher than on placebo (26.3 +/- 7.8 mmHg), difference not statistically significant (P = 0.1756, ANOVA). Area under curve of pain intensity rated at each distension step was significantly lower on asimadoline (89.3 +/- 33.9, ANOVA) than on placebo (108.1 +/- 29.7) (P = 0.0411). Thresholds of perception of nonpainful distensions were not altered on asimadoline, as compared with placebo. Colonic compliance was not different on placebo and asimadoline.

CONCLUSION

Asimadoline decreases overall perception of pain over a wide range of pressure distension of the colon in irritable bowel syndrome patients, without altering its compliance. These data suggest that further studies should explore the potential benefit of asimadoline in treatment of pain in irritable bowel syndrome patients.

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.

Evolving concepts in functional gastrointestinal disorders: promising directions for novel pharmaceutical treatments

In recent years there has been an increasing appreciation of the complexity of functional gastrointestinal disorders. These represent a spectrum of conditions which may affect any part of the gastrointestinal tract in which there appears to be dysregulation of visceral function and afferent sensation and a strong association with emotional factors and stress. There is a clear psychological dimension, with up to 60% of irritable bowel syndrome (IBS) patients reported to have psychological co-morbidities and altered pain perception is also common in comparison with control populations. The role of the enteric nervous system, the sensory pathways and the brain as well as the influence of the latter on sympathetic and parasympathetic outflow have likewise attracted increasing interest and have led to exciting new methods to study their complex interactions. The concept of low-grade inflammation, such as might occur after infection, acting as a trigger for neuromuscular dysfunction has also led to the broad integrative hypotheses that help to explain the biopsychosocial dimensions seen in functional gastrointestinal disease. The multi-component model places a major emphasis on neurogastroenterology and enteric and neuro-immune interactions where new approaches to pharmacotherapy lie. Drugs may affect motility, visceral sensation and other aspects of gut function such as secretion or absorption. More particularly, however, has been the search for and attempts to influence important mediators of these primary gut functions. Such targets include serotonin and selected 5-HT receptors, which are involved in gut motility, visceral sensation and other aspects of gut function, CCK receptors which are involved in the mediation of pain in the gut and nociception in the CNS, opioid receptors involved in pain in the brain, spinal cord and periphery, muscarinic M3-receptors, substance P and neurokinin A and B receptors which are involved in motor adaptation and pain transmission in association with inflammation, gabba receptors involved in nociception and cannabinoid receptors which are involved in the control of acetyl choline release in the gut. With a better understanding of the structures and pathways involved in visceral perception and hyperalgesia, in the CNS, spinal cord and the gut and new pharmacological tools we will be better able to elucidate the neuropharmacology of visceral perception and its relationship to gut dysfunction. It is likely that there will be multiple therapeutic options based on the spectrum of abnormalities capable of causing the spectrum of symptoms of functional gastrointestinal disorders in any individual patient.

Long-lasting changes in central nervous system responsivity to colonic distention after stress in rats

BACKGROUND & AIMS

The highly prevalent functional gastrointestinal disorders involve visceral pain and disturbed bowel habit and are associated with preceding stressful experiences, although causality and biological mechanisms remain unclear. The aim of the present study was to establish whether stress can directly and lastingly alter central nervous system responsivity to colonic distention in the rat as well as which neural pathways are likely to be involved.

METHODS

Rats were treated with a brief session of stressful foot shocks known to induce long-term behavioral and autonomic sensitization. Two weeks later, after induction of inhalation anesthesia, a balloon catheter was inserted in the distal colon and repeatedly inflated with brief, constant-pressure air pulses.

RESULTS

Reflex decreases in blood pressure and heart rate indicative of visceral afferent activation were greater in previously shocked rats than in controls. Colonic distention increased the expression of Fos, a marker of neuronal activation, in the sacral spinal cord and caudal brain stem. In the central amygdala and several cortical areas (prelimbic, infralimbic, agranular insular, cingulate), previously shocked rats showed reduced Fos expression following colonic distention compared with relevant controls.

CONCLUSIONS

The results indicate that a brief but intense stressful experience causes long-lasting alterations in higher-order central nervous system responsivity to colonic distention even in the absence of conscious affective responses, pointing to basic alterations in the neural pathways involved.

Pregabalin (CI-1008) inhibits the trinitrobenzene sulfonic acid-induced chronic colonic allodynia in the rat

In human, digestive disorders are often associated with visceral pain. In these pathologies, visceral pain threshold is decreased indicating a visceral hypersensitivity. Pregabalin [CI-1008; S-(+)-3-isobutylgaba] presents antihyperalgesic actions in inflammatory somatic pain models. This study was designed to evaluate 1) the effect of injection of TNBS into the colon on visceral pain threshold, and 2) the antihyperalgesic effect of pregabalin on TNBS-induced chronic colonic allodynia. A significant decrease in the colonic pain threshold was observed in trinitrobenzene sulfonic acid (TNBS)-treated animals (17.8 +/- 1.27 versus 43.4 +/- 1.98 mm Hg). Pregabalin (30-200 mg/kg s.c.) and morphine (0.1-1 mg/kg s.c.) showed a dose-related inhibition of TNBS-induced colonic allodynia. Pregabalin did not inhibit the colonic inflammatory effect of TNBS. In normal conditions (control animals), morphine (0.3 mg/kg s.c.) significantly increased the colonic pain threshold, whereas pregabalin (200 mg/kg s.c.) did not modify the colonic pain threshold. Pregabalin suppressed the TNBS-induced colonic allodynia but did not modify the colonic threshold in normal conditions. The ability of pregabalin to block the chronic colonic allodynia indicates that it is effective in abnormal colonic hypersensitivity, suggesting a possible effect in chronic pain in irritable bowel syndrome.

Vagotomy prevents morphine-induced reduction in Fos-like immunoreactivity in trigeminal spinal nucleus produced after TMJ injury in a sex-dependent manner

Acute injury to the temporomandibular joint (TMJ) region activates neurons in multiple, but spatially discrete, areas of the trigeminal spinal nucleus as seen by an increase in Fos-like immunoreactive neurons (Fos-LI). Pretreatment with morphine greatly reduces Fos-LI produced in the dorsal paratrigeminal area (dPa5), ventrolateral pole of the subnucleus interpolaris/caudalis (Vi/Vc-vl) transition region, and laminae I-II at the subnucleus caudalis/upper cervical cord junction (Vc/C2) suggesting a role for these areas in processing pain signals from the TMJ region. To determine if vagal afferents contribute to neural activation after TMJ injury or reduction of activity after morphine, Fos-LI was quantified in the lower brainstem and upper cervical spinal cord of intact and vagotomized male and female rats under barbiturate anesthesia. Bilateral cervical vagotomy (VgX) did not affect Fos-LI produced by TMJ injury in males or females in the absence of morphine. By contrast, morphine-induced reduction in Fos-LI produced at the Vi/Vc-vl transition region was prevented by prior VgX in males and diestrus females, but not in proestrus females. Morphine inhibition of Fos-LI produced in laminae I-II at the Vc/C2 junction region was diminished in vagotomized males compared to intact animals, but not affected in females. In an autonomic control area, the caudal ventrolateral medulla (CVLM), VgX reversed the morphine-induced reduction in Fos-LI in males and females similarly compared to their respective intact controls. These results were consistent with the hypothesis that the Vi/Vc-vl transition region plays a unique role in deep craniofacial pain processing and may integrate autonomic and opioid-related modulatory signals in a manner dependent on sex hormone status.

Pharmacology and clinical experience with fedotozine

Fedotozine [(1R)-1-phenyl-1-[(3,4,5-trimethoxy)benzyloxymethyl]-N,N- dimethyl-n-propylamine, (2S,3S-tartrate] is derived from the arylacetamide series. As with other compounds of this series, fedotozine is more or less selective of kappa(1)-opioid receptors and particularly for the kappa(1a)-receptor subtype, where it acts as an agonist. Pharmacological studies have shown that fedotozine exerts a peripheral antinociceptive action, comparable with that of other kappa-agonists. Its main effects have been demonstrated at the level of the afferent nerve pathways originating from the gut. Fedotozine alters the processing of visceral sensations along these pathways and hence, the perception of gut stimuli at the brain level. It modifies reflexes induced in various pathological conditions, like experimental inflammation of the gut, chemically-induced peritonitis or post-operative ileus. Fedotozine also decreases the nociceptive reflexes triggered by noxious gut distension in animals. In humans, fedotozine decreases the perception of gut distension, both in physiological and pathological conditions. Clinical trials undertaken in patients with functional digestive disorders, non-ulcer dyspepsia and irritable bowel syndrome, have shown that fedotozine relieves abdominal pain in these patients in 6-week treatments. kappa-Opioid receptors remain an interesting area for future development of new treatments for abdominal pain in patients with functional digestive disorders.

Opioid-receptor-mediated excitation of rat mesenteric afferent fibres supplying the rat jejunum

The aim of the present study was to examine the sensitivity to opioid-receptor agonists of mesenteric afferents supplying the small intestine and to characterize the subpopulations of any responsive fibres. Mesenteric afferent discharge was recorded electrophysiologically in response to cumulative doses (1-400 microgram kg-1) of the mu-receptor agonist [D-ala,2 N- me-Phe4, Gly5-ol]-enkephalin (DAMGO), the delta-receptor agonist [D-ala,2 D-leu5]-enkephalin (DADLE) and the kappa-receptor agonist U-50488. DAMGO and DADLE, but not U-50488, markedly stimulated whole nerve mesenteric afferent discharge (P < 0.05) that was unrelated to intestinal motor events. Subpopulations of afferent fibres responding to DAMGO were examined using waveform analysis to identify single units from within the whole mesenteric nerve bundles. One population was CCK-sensitive (15/15 fibres) and the other was a subpopulation of mechanosensitive afferents that responded to distension (17/28). 5-HT-sensitive afferents did not respond to DAMGO (0/11). We conclude that specific subpopulations of mesenteric afferents respond to mu- and possibly delta- but not kappa-receptor agonists. This sensitivity to opioids may contribute to the antinociceptive property of vagal afferents.

Abnormal cerebral processing of oesophageal stimuli in patients with noncardiac chest pain (NCCP)

In noncardiac chest pain (NCCP), altered visceral perception may result from abnormal cerebral processing of sensory input rather than abnormalities of afferent pathways. However, the interactions between symptoms, autonomic function and oesophageal stimuli are poorly studied. Oesophageal stimulation elicits reproducible cortical evoked potentials [CEP] and modulates heart rate variability via vagal pathways, as visible on power spectrum analysis of heart rate variability [PS-HRV]. These methods are increasingly used to study the function of visceral afferent neural pathways in human. The aim of this study was to compare EP and PS-HRV during oesophageal stimuli in NCCP and controls. Twelve healthy volunteers (one female, 11 male; aged 24-51 years; mean 32 +/- 8 years), and eight NCCP patients (three female, five male; age range 26-58, mean 40.5 +/- 10 years) were studied. Electrical oesophageal stimulation (EOS; 200 microseconds, 0.2 Hz, 25 stimuli) was applied to the oesophageal wall 5 cm above the lower oesophageal sphincter (LOS), and perception thresholds (measured in mA) determined. EP responses were recorded using 22 standard electroencephalogram scalp electrodes. Autonomic activity was assessed using PS-HRV, before, during, and after oesophageal stimulation. Measured PS-HRV indices included high frequency (HF; 0. 15-0.5 Hz) and low frequency (LF; 0.06-0.15 Hz) power, respectively, assessing vagal and sympathetic activity, and the LF/HF ratio. EOS perception occurred at lower thresholds in NCCP than in controls (3. 6 +/- 1 vs. 7.8 +/- 2 mA, P < 0.05). EP amplitude was greater (13 +/- 2 vs. 6 +/- 1 microV, P < 0.0001), and latency longer in controls vs. NCCP (191 +/- 7 ms vs. 219 +/- 6 ms, P < 0.001). In NCCP, EOS decreased sympathetic outflow (low frequency peak on PS-HRV) and increased cardiovagal activity (high frequency peak, P < 0.02) to a significantly higher degree in comparison with controls. During EOS, heart rate decreased in NCCP from 68 vs. 62 beats min-1 (P < 0.003) but not in controls. In NCCP patients, EOS was perceived at lower intensities and was associated with a greater cardiovagal reflex response. EP responses associated with EOS were smaller in NCCP than in controls, suggesting that an increased perception of oesophageal stimuli results from an enhanced cerebral processing of visceral sensory input in NCCP, rather than from hyperalgesic responses in visceral afferent pathways.

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.

The effect of fentanyl, DNQX and MK-801 on dorsal horn neurones responsive to colorectal distension in the anaesthetized rat

Certain dorsal horn neurones respond in a graded manner to noxious colorectal distension (CRD). Morphine inhibits these responses in the spinalized rat, but the role of excitatory amino acids in baseline visceral nociceptive transmission is less clear. This study examines the effect of the mu-opiate receptor agonist fentanyl, and the non-NMDA and NMDA antagonists DNQX and MK-801, respectively, on such responses to CRD in the sodium pentobarbitone-anaesthetized rat. Male rats were prepared for extracellular recording from the lumbosacral spinal cord. 90 neurones responsive to CRD, located throughout the dorsal horn, were classified according to their response duration and latency to 60 mmHg distension, as SL-A (short latency-abrupt; 59%), SL-S (short latency-sustained; 23%), L-L (long-latency; 10%) and Inhib (inhibited; 8%). Convergent cutaneous receptive fields were mapped for 79/90 neurones and classified as LT (low threshold), WDR (wide dynamic range) or HT (high threshold). CRD (20-100 mm Hg) elicited graded responses in most neurones. In 6/6 SL-S neurones, fentanyl (1-8 microg kg-1) dose-dependently inhibited the response to 60 mm Hg CRD, in a naloxone-sensitive manner, with an ID50 value (+/-95% confidence limits) of 2.48 (1.7-3. 7) microg kg-1. In 6/6 SL-A neurones, fentanyl had no significant effect on the response to CRD. DNQX (0.03-3 mg kg-1) produced a dose-dependent inhibition of the response to CRD in 5/5 SL-A neurones, with an ID50 value of 0.32 (0.01-41.1) mg kg-1. MK-801 (0. 03-0.3 mg kg-1) had no significant effect on responses to CRD in 6/6 SL-A neurones. The differential inhibitory effects of fentanyl on two neuronal subtypes may indicate functional differences. In SL-A neurones AMPA/kainate, but not NMDA receptors are involved in mediating baseline nociceptive neurotransmission.

Differential effects of mu-, delta-, and kappa-opioid receptor agonists on mechanosensitive gastric vagal afferent fibers in the rat

Single-fiber recordings were made from the decentralized right cervical vagus nerve (hyponodosal) of the rat. A total of 56 afferent fibers that responded to gastric distension (GD) were studied: 6 fibers were stimulated by phasic balloon GD, 50 by fluid GD. All fibers gave increasing responses to increasing pressures of GD (5-60 mmHg). The effects of mu-opioid (morphine), delta-opioid (SNC80), and kappa-opioid (EMD61,753, U62,066) receptor agonists were tested on responses of afferent fibers to GD. Morphine, administered systemically over a broad dose range (10 microg to 31 mg/kg, cumulative), had no effect on either resting activity or responses of vagal afferent fibers to GD. Similarly, the delta-opioid receptor agonist SNC80 (0.05-3.2 mg/kg) did not affect resting activity or responses to GD. In contrast, cumulative intra-arterial doses of the kappa-opioid receptor agonist EMD61,753 or U62,066 dose dependently attenuated afferent fiber responses to GD. Doses producing inhibition to 50% of the control response to GD of EMD61,753 (8.0 mg/kg) and U62,066 (8.8 mg/kg) did not differ. The effect of U62,066 was moderately attenuated by a nonselective dose (4 mg/kg) of naloxone hydrochloride; the kappa-opioid receptor-selective antagonist nor-BNI (20 mg/kg) was ineffective. These results demonstrate that kappa-, but not mu- or delta-opioid receptor agonists modulate visceral sensation conveyed by vagal afferent fibers innervating the stomach. Given that kappa-opioid receptor agonists effects were only modestly antagonized by naloxone and not at all by nor-BNI, the results point to a novel site of action.

Kappa opioid receptor agonists differentially inhibit two classes of rat spinal neurons excited by colorectal distention

BACKGROUND & AIMS

Quantitative neurophysiological studies have identified the presence of at least 2 spinal neuronal populations (abrupt and sustained) that are excited by the noxious visceral stimulus colorectal distention. This study examined the effects of the kappa opioid receptor agonists fedotozine and U50488H on the activity of these neurons.

METHODS

In decerebrate, cervical spinal cord-transected male rats, the lumbosacral spinal cord was exposed by a laminectomy. Dorsal horn neurons showing excitatory responses to colorectal distention (80 mm Hg, 20 seconds) were identified using microelectrodes. Cumulative doses of fedotozine and U50488H were administered intravenously or intrathecally, and antagonists were used.

RESULTS

Intravenous fedotozine and U50488H dose-dependently inhibited the evoked activity of sustained neurons. This inhibition was partially reversed by the kappa opioid antagonist norbinaltorphimine. The same agents had insignificant effects on the evoked activity of abrupt neurons. Fedotozine inhibited spontaneous activity of both abrupt and sustained neurons. Intrathecally administered U50488H had no effect on abrupt or sustained neurons, but intrathecally administered fedotozine inhibited the evoked and spontaneous activity of both groups.

CONCLUSIONS

Kappa opioid receptor agonists acting peripherally had differential effects on 2 spinal neuronal populations responsive to colorectal distention. Fedotozine had additional inhibitory effects acting within the spinal cord.

Influence of trimebutine on inflammation- and stress-induced hyperalgesia to rectal distension in rats

The effects of trimebutine and its major metabolite, N-desmethyltrimebutine on inflammation- and stress-induced rectal hyperalgesia have been evaluated in rats fitted with electrodes implanted in the longitudinal striated muscle of the abdomen. Intermittent rectal distension was performed before and 3 days after induction of rectal inflammation by local infusion of trinitrobenzenesulphonic acid (in ethanol). Stress consisted of 2h partial restraint and rectal distension was performed before and 30min after the end of the partial restraint session. The animals were treated intraperitoneally with trimebutine or desmethyltrimebutine (5, 10 or 20mgkg(-1)) or vehicle 15min before rectal distension. Naloxone (1mgkg(-1)) or saline was injected subcutaneously before trimebutine and desmethyltrimebutine. Before treatment trimebutine at the highest dose (20mgkg(-1)) reduced the abdominal response to rectal distension for the highest volume of distension (1.6mL) whereas desmethyltrimebutine was inactive. After rectocolitis the abdominal response to rectal distension was enhanced and trimebutine at 5mgkg(-1) reduced and at 10 mgkg(-1) suppressed inflammation-induced hyperalgesia, an effect reversed by naloxone. Desmethyltrimebutine was inactive. Stress-induced hypersensitivity was attenuated or suppressed, or both, by trimebutine and desmethyltrimebutine at doses of 5, 10 or 20mgkg(-l); greater efficacy was observed for desmethyltrimebutine and the effects were not reversed by naloxone. It was concluded that trimebutine and desmethyltrimebutine are active against inflammation- and stress-induced rectal hyperalgesia but act differently. The effect of trimebutine on inflammation-induced hyperalgesia is mediated through opioid receptors.

Antinociceptive effects of morphine and U-50,488H on vaginal distension in the anesthetized rat

The antinociceptive activity of the kappa- and mu-opioid receptor agonists, (+/-)-U-50,488H and morphine, was examined in a vaginal distension model in anaesthetized female rats. Vaginal distension induced a reproducible cardiovascular response (CVR) which was inhibited in a dose related manner by morphine (0.03-1.0 mg/kg i.v., ED50 = 0.16 mg/kg) and (+/-)-U-50,488H (0.08-1.6 mg/kg i.v., ED50 = 0.49 mg/kg). Morphine (0.3 microg/rat) administered i.c.v. inhibited the CVR by 81.6 +/- 7.9% whereas (+/-)-U-50,488H (30-300 microg/rat) was inactive by this route. A low dose of naloxone (30 microg/kg i.v.) blocked the effect of morphine but not that of (+/-)-U-50,488H. The kappa-opioid antagonist, nor-binaltorphimine (10 mg/kg s.c.) abolished the response to (+/-)-U-50,488H but not that of morphine. This demonstrates that both central and peripheral mu-opioid receptors may be involved in morphine-induced antinociception whereas the kappa-opioid agonist, (+/-)-U-50,488H, blocks vaginal nociception by acting on peripheral kappa-opioid receptors.

Cellular localization and distribution of the cloned mu and kappa opioid receptors in rat gastrointestinal tract

Several pharmacological and electrophysiological studies have shown that the opioid receptors are widely distributed in the gastrointestinal tract. Despite such consensus, there are conflicting findings regarding their effects in intestinal function, and their precise site of action remained unclear. The aim of the present study was therefore to delineate the cellular localization of mu and kappa opioid receptors in rat gastrointestinal tract using polyclonal antibodies generated to C-terminal end of the cloned mu (63 amino acids) and kappa (41 amino acids) receptors. The distribution of mu differs from that of kappa receptors within the gastrointestinal wall, with a greater abundance of mu receptor-like immunoreactive fibres in all intestinal layers. Numerous neurons expressing mu receptor-like proteins were found in the submucosal plexus with comparatively few in the myenteric plexus. In contrast, a higher number of neurons expressing kappa receptor-like immunoreactivity were visualized in the myenteric plexus with a small number in the submucosal plexus. A high number of immunopositive neurons were found in the myenteric plexus of the stomach and the proximal colon with both antibodies. In the submucosal and mucosal layers. mu receptor-immunoreactive fibres were more abundant and distributed around the crypts, blood vessels and lymphatic nodes. Interestingly, numerous mu and fewer kappa receptor-immunoreactive interstitial cells are localized in the region of myenteric plexus and at the internal border of the circular muscle. Finally, smooth muscle cells did not demonstrate any mu- nor kappa-receptor immunoreactivity. These findings suggest that in the rat gastrointestinal tract, mu and kappa opioid receptors may directly influence neuronal and interstitial cell activity. This appears not to be the case for the smooth muscle cells. In the muscular layers, the anatomical data point to mu receptor actions being mediated by nerve terminals, whereas kappa receptor effects may be mediated by both nerve terminals and somatodendritic synaptic mechanisms. In contrast, in the submucosal and mucosal layers, mu receptors predominate and are localized on both nerve terminals and somatodendritic synaptic elements.

Intravenous versus intraperitoneal morphine before surgery to provide postoperative pain relief

BACKGROUND

Opioid receptors have been demonstrated on peripheral afferent nerves throughout the body. The aim of the present study was to compare the effects of intravenous and intraperitoneal administration of morphine with regard to pain, postoperative morphine requirement, and recovery after major abdominal surgery, and to describe the pharmacokinetics of intraperitoneal morphine in humans.

METHODS

In a double-blind manner, 30 patients scheduled for major abdominal surgery were randomized to either 50 mg of morphine intravenously (i.v.) or 50 mg of morphine intraperitoneally (i.p.) before operation. Pain was measured on a visual analogue scale and morphine requirements were registered for 3 days. Recovery was measured as time to oral intake of food, time to flatulence and days in hospital. Plasma morphine, morphine-3-glucuronide, and morphine-6-glucuronide concentrations were determined during the first 4 h after morphine administration.

RESULTS

During the first postoperative hours there was less pain at rest (P = 0.02) and on coughing (P = 0.004) in the intravenous group. The requirement of additional morphine (P = 0.016) was lower in the intravenous group during the first postoperative day. No major differences in recovery were seen. The plasma concentrations of morphine measured as area under the curve (AUC) during the first 4 h were similar, but the intravenous group showed significantly higher concentrations of the active metabolite morphine-6-glucuronide, (P = 0.016), indicating a difference in pharmacokinetics after intraperitoneal compared to intravenous administration of morphine.

CONCLUSION

Intraperitoneal administration of 50 mg of morphine before major abdominal surgery is less efficient in reducing pain and postoperative morphine requirements than the same amount of morphine given intravenously.

Involvement of prostaglandins and CGRP-dependent sensory afferents in peritoneal irritation-induced visceral pain

This study investigates the contribution of prostaglandins (PG) and calcitonin gene-related peptide (CGRP) pathways in visceral pain induced by peritoneal irritation in rats. Peritoneal irritation was produced by i.p. administration of acetic acid (AA: 0.06-1.0%, 10 ml/kg). Visceral pain was scored by counting abdominal contractions. The effect of CGRP (3-100 microg/kg, i.p.) was also evaluated. Like AA, CGRP induced abdominal pain. Neonatal pretreatment with capsaicin reduced abdominal contractions produced by AA (0.6%) and CGRP (20 microg/kg) with 64.6% and 45.6%, respectively. Abdominal contractions induced by AA and CGRP were blocked by two antinociceptive drugs, mu-and kappa-opioid agonists, morphine and (+/-)-U-50,488H, respectively. Indomethacin (3 mg/kg, s.c.) reduced the number of abdominal contractions produced by AA by 78.1%+/-6.4% but did not inhibit abdominal contractions produced by CGRP. The CGRP, receptor antagonist, hCGRP(8-37) (300 microg/kg, i.v.) inhibited AA- and CGRP-induced abdominal contractions with 57.5%+/-12.4% and 51.6%+/-11.3%, respectively. Concomitant i.p. administration of PGE1 and PGE2 (0.3 mg/kg of each) produced abdominal contractions which were inhibited 45.6%+/-9.3% by hCGRP(8-37) (300 microg/kg i.v.). Taken together, these results suggest that peritoneal irritation is likely to trigger the release of prostaglandins, which in turn produces a release of CGRP from primary sensory afferents.

Fedotozine blocks hypersensitive visceral pain in conscious rats: action at peripheral kappa-opioid receptors

The effect of fedotozine on visceral hypersensitivity was evaluated in conscious rats. One hour after colonic irritation (0.6% acetic acid intracolonically), a 30 mmHg colonic distension was applied for 10 min. Irritation increased the number of abdominal contractions induced by colonic distension (23.4 +/- 4.1 versus 4.8 +/- 1.4 in saline-treated rats, P < 0.001). Facilitation of colonic pain was reversed in a dose-dependent manner by fedotozine ((+)-(-1R1)-1-phenyl-1-[(3,4,5-trimethoxy)benzyloxymethyl]-N ,N-dimethyl-n-propylamine), (+/-)-U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-1-pyrrolidinyl]cyclohexyl)benzen eacetamide) and morphine (respective ED50 values 0.67, 0.51 and 0.23 mg/kg s.c.). The kappa-opioid receptor antagonist, nor-binaltorphimine, abolished the effects of fedotozine and (+/-)-U-50,488H but not those of morphine. Low doses of naloxone (30 microg/kg s.c.) blocked the effect of morphine but not of fedotozine or (+/-)-U-50,488H. After intracerebroventricular administration, morphine was very potent (ED50 1.7 microg/rat), (+/-)-U-50,488H poorly active (58% of antinociception at 300 microg/rat) and fedotozine inactive up to 300 microg/rat. These results show that fedotozine blocks hypersensitive visceral pain by acting on peripheral kappa-opioid receptors in animals.

Effects of opioids on mechanosensitive pelvic nerve afferent fibers innervating the urinary bladder of the rat

A total of 443 pelvic nerve afferent fibers in the L6 dorsal root of the rat were identified by electrical stimulation of the pelvic nerve; 319 (72%) were myelinated A delta fibers with a mean conduction velocity (CV) of 11.8 m/s and 124 (28%) were unmyelinated C fibers with mean CV of 1.9 m/s. Two hundred fifty-two fibers (57%) responded to noxious urinary bladder distension (UBD; 80 mmHg); 108 were C fibers (mean CV; 1.9 m/s) and 144 were A delta fibers (mean CV; 8.2 m/s). Forty-nine UBD-sensitive fibers were further characterized; all gave monotonic increases in firing to increasing distending pressures. Thirty-six fibers (73%) had a low-threshold (LT) for response (mean: 6 mmHg) and 13 fibers (27%) had high-thresholds (HT) for response (mean: 32 mmHg). Responses of 15 fibers to graded UBD (11 LT and 4 HT) were tested before and after instillation of 0.5 ml of 30% xylenes (n = 11) or 5% mustard oil (n = 4) into the bladder. The mean resting activity of 13 fibers significantly increased, and 7 fibers exhibited sensitization of responses to graded UBD 30 min after xylenes or mustard oil instillation. All 4 HT fibers were sensitized; 3 of the 11 LT fibers were sensitized (i.e., gave increased responses to UBD). The effects of opioid receptor agonists were tested on responses to noxious UBD (80 mmHg). Cumulative intraarterial doses of mu-opioid receptor agonists (morphine, 8 mg/kg, and fentanyl, 300 micrograms/kg) and of delta-opioid receptor agonists (DPDPE, 300 micrograms/kg, and SNC-80, 300 micrograms/kg) did not affect responses to noxious UBD. In contrast, cumulative 16 mg/kg intraarterial doses of the kappa-opioid receptor agonists U50,488H, U69,593 and U62,066 dose-dependently attenuated responses to noxious UBD. There were no differences in the dose-response relationships of these drugs on afferent fibers from untreated and xylenes- or mustard oil-treated urinary bladder. These results reveal that there is a greater proportion of UBD-sensitive fibers in the L6 dorsal root (57%) than in the S1 dorsal root of the rat (38%; a previous study). The attenuation of responses to UBD by kappa, but not mu or delta opioid receptor agonists suggests a potential use for peripherally acting kappa opioid receptor agonists in the control of urinary bladder pain.

Inhibitory effects of various spasmolytics on the vagal afferent gastric excitatory response in cats

The inhibitory effects of atropine, cimetropium, pirenzepine and N-butylscopolamine on the vagal afferent gastric excitatory response in cats under anesthesia with pentobarbital sodium and infusion of gallamine were examined. Electrical stimulation of vagal trunk in left side (10 Hz in frequency, 3 msec in duration, 15 V in intensity and for 10 sec) caused an initial gastric excitatory response during the period of stimulation followed by a late excitatory gastric response after stimulation in normal cats. The initial response was inhibited by atropine (100 microg/kg, i.v.) and hexamethonium (10 mg/kg, i.v.), while the late response was inhibited by atropine but not by hexamethonium (10 mg/kg, i.v.). In chronic supranodose vagotomized cats 11-15 days after the operation, stimulation of the vagal trunk caused a late gastric excitatory response after the stimulation period, which was inhibited by atropine (100 microg/kg, i.v.) but not by hexamethonium (10 mg/kg, i.v.). The two types of gastric responses in normal cats have been defined as follows: the initial gastric excitatory response (atropine- and hexamethonium- sensitive) is due to activation of vagal efferent fibers and the late gastric excitatory response (atropine-sensitive and hexamethonium-resistant) is due to activation of vagal afferent fibers. ED50 values of atropine, cimetropium, pirenzepine and N-butylscopolamine in inhibiting the vagal afferent gastric response were 7.2 microg/kg (n=4), 2.4 microg/kg (n=6), 82.6 microg/kg (n=3) and 93.0 microg/kg (n=4), respectively. The inhibitory effects of atropine and cimetropium on the vagal afferent gastric excitatory response (hexamethonium-resistant) were more potent than those of pirenzepine and N-butylscopolamine. These results suggested that the potent inhibitory effects of cimetropium and atropine on the vagal afferent gastric response may involve a potent spasmolytic effect.

Reversal by kappa-agonists of peritoneal irritation-induced ileus and visceral pain in rats

Peritoneal irritation in rats induced by i.p. administration of acetic acid produces abdominal contractions reflecting visceral pain, and gastrointestinal ileus characterized by inhibition of gastric emptying and small intestine transit. In this study, gastric emptying (GE) and intestinal transit, calculated by the geometric center (GC) method, were estimated using a test meal labeled with 51Cr-EDTA. Visceral pain was assessed by counting abdominal contractions. Acetic acid produced abdominal contractions (80.8 +/- 3.3) and inhibition of GE (-54%) and GC (-63%) during the test-period. The kappa-opioid receptor agonists, CI-977 (+/-)-U-50,488H, (+/-)-bremazocine, PD-117,302, (-)-cyclazocine, and U-69,583, reversed abdominal contractions and inhibitions of gastrointestinal transit in a dose-related manner. The mu-opioid receptor agonists and potent analgesics, morphine and fentanyl did not restore normal gastric emptying and intestinal transit. These data suggest that selective kappa-opioid receptor agonists might be used to treat abdominal pain associated with motility and transit impairment during postoperative ileus.

Attenuation of c-Fos expression in the rat lumbosacral spinal cord by morphine or tramadol following noxious colorectal distention

We have previously reported that repetitive, noxious colorectal distention (CRD) induces c-Fos in the lumbosacral spinal cord. This study examined the effects of the analgesics morphine and tramadol on c-Fos expression resulting from noxious CRD in the rat. Pre-treatment (30 min or 1 min, i.v.) with morphine (1.25 mg/kg-5.0 mg/kg) or tramadol (1 mg/kg-20 mg/kg) dose-dependently attenuated c-Fos expression to CRD in all areas of the L6-S1 spinal gray matter. The highest dose of morphine was equipotent to the highest dose of tramadol. Repetitive dosing (1/4 of the greatest dose every 30 min) was as effective as a single bolus dose for both drugs. The visceromotor response to CRD was dose-dependently attenuated by tramadol and was reversed by naloxone. However, the dose of tramadol that eliminated the visceromotor response (7% of control) reduced the c-Fos expression to 47% of control. These results demonstrate that these two analgesics attenuate immediate-early gene expression and the visceromotor response to a noxious visceral stimulus and suggest that complete attenuation of c-Fos expression is not necessary for these compounds to produce analgesia to a noxious visceral stimulus.

Oxymorphone-induced analgesia and colonic motility measured in colorectal distension

Changes in colonic motility in rats following intravenous (IV) oxymorphone (0.1 mg/kg), atropine (0.1 mg/kg), or saline were monitored to determine whether opioid-induced changes in colonic motility affect antinociceptive measurements when using colorectal distension (CRD) as a nociceptive assay. Polygraph recordings of colonic pressures, contraction frequencies, and the pressure-volume relationship of the stimulus showed that oxymorphone produced a transient increase in contraction frequencies when compared to atropine- and saline-treated rats. The transient increase in contraction frequency caused by oxymorphone declined to baseline levels at 30 min after administration, the time at which the nociceptive threshold for CRD was tested. Neither oxymorphone nor atropine changed baseline pressures or the pressure-volume curve for the balloon stimulus. Antinociceptive results from CRD at 30 min posttreatment showed that only oxymorphone produced significant antinociception. We conclude that oxymorphone does not produce changes in colonic motility that complicate antinociceptive measurements in CRD and that CRD is an effective means of testing opioid-induced visceral antinociception.

Spinal mu and delta, but not kappa, opioid-receptor agonists attenuate responses to noxious colorectal distension in the rat

The antinociceptive efficacy of different opioid-receptor agonists following their intrathecal (i.t.) administration was examined in awake, unanesthetized rats in a model of visceral pain. Cumulative i.t. doses of the mu-preferring opioid-receptor agonist morphine produced dose-dependent attenuation of the change (increase) in mean arterial pressure (delta MAP) and elevation of the visceromotor threshold to colorectal distension (CRD). Similar dose-dependent antinociceptive effects were produced after i.t. administration of the mu opioid-receptor-selective agonist DAMPGO. Morphine and DAMPGO were equipotent against the delta MAP to phasic CRD (80 mm Hg, 20 sec), but DAMPGO was more than 6 times more potent than morphine in elevating the visceromotor threshold to an incrementing CRD. Intrathecal administration of the delta opioid-receptor-selective agonist DPDPE produced, like morphine and DAMPGO, a dose-dependent attenuation of the delta MAP to CRD; DPDPE was one-tenth as potent as morphine or DAMPGO. DPDPE also dose-dependently elevated the visceromotor threshold to CRD, but its efficacy was only half that of morphine or DAMPGO. The kappa opioid-receptor-selective agonist U 50488H was without antinociceptive efficacy after i.t. administration, but did attenuate responses to CRD after systemic administration. The antinociceptive effects produced by morphine and DAMPGO were antagonized by i.t. pretreatment with naloxone and the effects produced by DPDPE were antagonized by i.t. pretreatment with the delta opioid-receptor-selective antagonist naltrindole. These data indicate that local mu and delta, but not kappa, opioid receptors can modulate visceral nociceptive transmission in the spinal cord.

Review article: the hypersensitive gut--peripheral kappa agonists as a new pharmacological approach

Hypersensitivity to pain is a common component of functional bowel disorders. Hyperalgesia may be induced by various stimuli which produce a cocktail of inflammatory mediators that decrease the pain threshold. Drugs able to block these peripheral events within the gut may offer a new pharmacological approach for treating functional bowel disorders. Kappa opioids have been shown to inhibit somatic pain through a peripheral mechanism of action, acting directly on receptors located on peripheral sensory endings. They can block both the nociceptive messages as well as the release of sensory peptides. This paper reviews the effects of opioid agonists on gut visceral pain and motility anomalies induced by visceral pain. Kappa opioids have strong effects on all models tested, with a peripheral mechanism of action allowing the design of drugs acting only in the periphery and having no central nervous system side-effects. This contrasts with mu agonists which are centrally active on pain and worsen the subsequent transit and motility anomalies.

Effect of fedotozine on the cardiovascular pain reflex induced by distension of the irritated colon in the anesthetized rat

The effect of fedotozine was evaluated in a model of colonic hypersensibility to balloon distension in anesthetized rats. Acetic acid (0.6%, intracolonically) significantly enhanced the hypotension reflex response to colonic distension (P < 0.05). At a noxious pain pressure (75 mm Hg), fedotozine ((+)-(-1R)-1-phenyl-1-[(3,4,5- trimethoxy)benzyloxymethyl]-N,N-dimethyl-n-propylamine) had no effect at 0.6 and 1 mg/kg i.v. in saline-treated rats and higher doses were required to produce antinociception (ED50 = 2.57 mg/kg i.v.). By contrast, fedotozine at 0.6 and 1 mg/kg i.v. displayed 38 and 54% antinociception (P < 0.05) respectively, in acetic acid-treated animals, leading to a decrease in its ED50 (1.15 mg/kg i.v.). Similar results were obtained with (+/-)-trans-N-methyl-N-[2-(pyrrolidinyl)-cyclohexyl]benzo[b]-thiophene- 4-acetamide (PD-117,302), a kappa-opioid receptor agonist, while the antinociceptive action of morphine and a kappa-opioid receptor agonist, trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1- pyrrolidinyl]cyclohexyl)benzenacetamide ((+/-)-U-50,488H), was identical in control and acetic acid-treated animals. Nor-binaltorphimine, a selective kappa-opioid receptor antagonist, reversed the enhanced antinociceptive activity of fedotozine and PD-117,302. It is concluded that acetic acid induces colonic hypersensibility to painful mechanical stimuli and that some but not all kappa-opioid receptor ligands can have enhanced efficacy in this pathological situation.

Peripheral kappa-opioid receptors mediate the antinociceptive effect of fedotozine (correction of fetodozine) on the duodenal pain reflex inrat

Fedotozine has been shown to act on gastrointestinal sensitivity through peripheral kappa-opioid receptors. The present study investigated the action of fedotozine and reference compounds, morphine and (+/-)-U-50,488H, on duodenal pain in anesthetized rats. The noxious stimulus was produced by duodenal distension (100 mm Hg; 30 s). Fedotozine (1-5 mg/kg i.v.) produced a dose-dependent inhibition of the cardiovascular reflex induced by duodenal distension (ED50 = 1.87 mg/kg) but had no effect at doses up to 300 micrograms/rat by either intracerebroventricular (i.c.v.) or intrathecal routes (i.t.). The mu-opioid receptor agonist, morphine, was active by both i.v. (ED50 = 0.62 mg/kg) and i.c.v. routes (ED50 = 2.17 micrograms/rat) as was the kappa-opioid receptor agonist, (+/-)-U-50,488H (trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1- pyrrolidinyl]cyclohexyl)benzeneacetamide) (ED50 = 0.25 mg/kg and 149 micrograms/rat for i.v. and i.c.v. routes, respectively). The selective kappa-opioid receptor antagonist, nor-binaltorphimine (10 mg/kg s.c.), abolished the response to fedotozine (5 mg/kg i.v.) and (+/-)-U-50,488H (2 mg/kg i.v.) but not that to morphine (1 mg/kg i.v.). In contrast, naloxone (30 micrograms/kg i.v.) blocked the response to morphine (1 mg/kg i.v.) but not that to fedotozine (5 mg/kg i.v.) or (+/-)-U-50,488H (2 mg/kg i.v.). It is concluded that the antinociceptive effects of fedotozine on duodenal pain are mediated by peripheral kappa-opioid receptors.